This study explores the causes that arise when foreign workers (E-9) introduced into Korea become unregistered foreign workers due to changes in their residency status. It systematically examines studies in domestic academic journals and examines the causes of unregistered foreign workers. Institutional and social factors were analyzed. We classified the factors that amplified the number of unregistered foreign workers that occurred during the 20 years since the Employment Permit System was implemented and discussed the implications of these factors. In addition, the flow of research by year was identified and classified into basic information, research content, and research method, and policy implications were derived by deriving results appropriate for the area. Accordingly, this study is expected to serve as basic data for system improvement in a reality where we are facing the need for social discussion regarding unregistered foreign workers.